1. Field of the Invention
The invention is generally related to pressure sensors and, more particularly, to a new kind of pressure sensor which utilizes a combination of an electric component that includes spaced apart conductive films with a liquid crystal therebetween and an optical component which includes a polarized light transmission scheme that operates in conjunction with the liquid crystal.
2. Description of the Prior Art
Pressure is defined as force per unit area. Precise knowledge of the pressure on a system is required in a number of situations. For example, in any system which involves fluid-dynamics (e.g., oil rigs, air conditioning equipment, etc.), monitoring and control of differential pressure is essential. Pressure is one of the fundamental terms in the ideal gas law and its corollaries and is one of the determining factors in vapor-liquid equilibria. In semiconductor manufacturing, many fabrication processes are performed under vacuum pressure (e.g., plasma etching, etc.) and knowledge and control of the pressure in these processes is absolutely required. Pressure often plays an important role in the safety and performance of a system or element (e.g., point of failure, etc.).
Over the years, many different sensors have been devised for measuring pressure. For example, diaphragm elements have been used as pressure sensors wherein a thin flexible disk is connected to an indicator pointer, recorder pen, or other element. Deflection of the diaphragm due to pressure from an impinging fluid or other source causes the indicator pointer, recorder pen, or other element to move on scale which is related to pressure. Diaphragm capsules are used in a similar pressure measurement scheme and include two or more circular diaphragms welded at the edges. An inverted bell pressure sensing device includes two inverted bells in an oil bath suspended on opposite ends of a balance beam. Movement of the balance beam due to different pressures under the two bells is coupled to an attached pointer or other indicating device which is scaled to correlate with pressure. A Bourdon tube pressure sensor includes a tube with a sealed end and an open end which is connected to a fluid pressure source. The tube is typically spiral shaped, helix shaped or a C-type. Rotating motion of the tube under the operating pressure actuates a pointer or other indicator that corresponds with a pressure scale. Piezoresistive transducers convert a change in pressure into a change in resistance. These type of pressure sensors are typically silicon wafers or chips with embedded resistors. Strain on the embedded resistor caused by a source of pressure on the silicon substrate results in a linear change in resistance that is measured and converted to pressure units. Piezoelectric pressure sensing devices are based on the principle that quartz, when properly cut and oriented with respect to its crystallographic axes, generates a small electric charge on certain surfaces when stressed. In practice, a stack of piezoelectric elements are coupled to a diaphragm element, such that deflection of the diaphragm under the influence of a pressure source results in a compressive force on the quartz stack which, in turn, generates a charge proportional to the stack.
The quantitative measurement of local and global pressure on a surface is important in many applications. For example, local and global pressure measurements are particularly required in a flow field on the walls of an object in aerodynamic testing. Conventional techniques of pressure measurement in such applications generally fall in two categories. First, pressure measurements can be performed where calibration of the device is independent of the flow. Second, pressure measurements can be performed where calibration of the device in terms of the law of the wall is necessary. Pressure sensors utilizing capacitance gauges, magnin wires, etc., can be calibrated against dead weights, and are exemplary of the first category of pressure measurement. Pressure scanners and Preston tubes rely on the basic Newtonian relations and thus produce results which are unreliable and difficult to interpret in a disturbed and/or under dynamic boundary layer conditions. Pressure sensitive paints, apart from having slow response controlled by diffusion times (on the order of a few seconds), suffer similarly from the requirement of an oxygen environment which is undesirable in some working situations and also have repeatability problems. Moreover, for local pressure measurements, there are practical limitations in microsizing most available sensors. Also, when sensors need to be mounted within the body of the surface being monitored, there are limitations on the area of measurements. In view of this, there is a clear need for development of pressure sensors capable of measuring local as well as global pressures on a continuous basis which are easy to mount and cost effective.